Along with the recent development of so-called information-oriented society, electronic apparatuses, such as personal computers and PDA (Personal Digital Assistants), have been widely used. With the spread of such electronic apparatuses, portable apparatuses that can be used in offices as well as outdoors have been used, and there are demands for small-size and light-weight of these apparatuses. Liquid crystal display devices are widely used as one of the means to satisfy such demands. Liquid crystal display devices not only achieve small size and light weight, but also include an indispensable technique in an attempt to achieve low power consumption in portable electronic apparatuses that are driven by batteries.
The liquid crystal display devices are mainly classified into the reflection type and the transmission type. In the reflection type liquid crystal display devices, light rays incident from the front face of a liquid crystal panel are reflected by the rear face of the liquid crystal panel, and an image is visualized by the reflected light, whereas in the transmission type liquid crystal display devices, the image is visualized by the transmitted light from a light source (backlight) placed on the rear face of the liquid crystal panel. Since the reflection type liquid crystal display devices have poor visibility because the reflected light amount varies depending upon environmental conditions, transmission type color liquid crystal display devices using color filters are generally used as display devices of personal computers for displaying multi-color or full-color images.
As the color liquid crystal display devices, TN (Twisted Nematic) type using switching elements such as a TFT (Thin Film Transistor) are widely used. Although the TFT-driven TN type liquid crystal display devices have better display quality, compared to STN (Super Twisted Nematic) type liquid crystal display devices, they require a backlight with high brightness to achieve high screen brightness because the light transmittance of the liquid crystal panel is only several percent or so at present. For this reason, a lot of power is consumed by the backlight. Moreover, since a color display is achieved using color filters, a single pixel needs to be composed of three sub-pixels, and there are problems that it is difficult to provide a high-resolution display, and the purity of the displayed colors is not sufficient.
In order to solve such problems, the present inventors developed field-sequential type liquid crystal display devices, (see, for example, T. Yoshihara, et. al., ILCC 98, P1-074, 1998; T. Yoshihara, et. al., AM-LCD '99 Digest of Technical Papers, p. 185, 1999; and T. Yoshihara, et. al., SID '00 Digest of Technical Papers, p. 1176, 2000, and the like). Such field-sequential type liquid crystal display devices do not require sub-pixels, and therefore, displays with higher resolution can be easily realized compared to color-filter type liquid crystal display devices. Moreover, since a field-sequential type liquid crystal display device can use the color of light emitted by the light source as it is for display without using a color filter, the displayed color has excellent purity. Furthermore, since the light utilization efficiency is high, a field-sequential type liquid crystal display device has the advantage of low power consumption. However, in order to realize a field-sequential type liquid crystal display device, high-speed responsiveness (2 ms or less) of liquid crystal is essential.
In order to provide a field-sequential type liquid crystal display device with significant advantages as mentioned above or increase the speed of response of a color-filter type liquid crystal display device, the present inventors are conducting research and development on the driving of liquid crystals such as a ferroelectric liquid crystal having spontaneous polarization, which may achieve 100 to 1000 times faster response compared to a prior art, by a switching element such as a TFT (for example, Japanese Patent Application Laid-Open No. 11-119189/1999, and the like). In the ferroelectric liquid crystal, the long-axis direction of the liquid crystal molecules tilts with the application of voltage. A liquid crystal panel sandwiching the ferroelectric liquid crystal therein is sandwiched by two polarization plates whose polarization axes are orthogonal to each other, and the intensity of the transmitted light is changed using birefringence caused by the change in the long-axis direction of the liquid crystal molecules.
As described above, the field-sequential type liquid crystal display device has higher light utilization efficiency and can reduce power consumption compared to the color-filter type liquid crystal display device. However, a further reduction in power consumption is required for portable apparatuses that are driven by batteries. Similarly, color-filter type liquid crystal display devices are required to reduce power consumption.
The following description will explain the display function, particularly a memory display function of a liquid crystal display device using a ferroelectric liquid crystal having a spontaneous polarization or the like. Such a liquid crystal display device has a normal display function that rewrites the displayed image at a predetermined cycle by applying a voltage to the liquid crystal, and a memory display function that stops the application of voltage to the liquid crystal and retains the image displayed before stopping the application of voltage. In the memory display function, after removing all voltages applied to the liquid crystal by switching elements such as TFT, the display state just before the removal of applied voltage is substantially retained, and therefore it is possible to display the image without applying a voltage to the liquid crystal material, thereby being capable of significantly reducing power consumption. Thus, such a liquid crystal display device is applicable to portable apparatuses, and has a significant effect of reducing power consumption, especially on portable apparatuses that often display still images.
The memory function of the ferroelectric liquid crystal having a spontaneous polarization is described below. A voltage is applied to a liquid crystal panel, and then the voltage is removed by stopping the application of voltage. The light transmittance during the application of voltage and the light transmittance at 60 seconds after the removal of the voltage are measured while changing the value of the applied voltage, and one example of the measurement results is shown in FIG. 1. FIG. 1 shows the measurement results by plotting the applied voltage (V) on the abscissa and the light transmittance (%) on the ordinate, wherein O-O represents the light transmittance during the application of voltage, and Δ-Δ represents the light transmittance at 60 seconds after the removal of the voltage. The corresponding applied voltage-light transmittance characteristics does not change even after the removal of applied voltage, and thus it can be understood that even when the voltage applied to the liquid crystal panel is removed, the light transmittance corresponding to the display state when the voltage is applied is maintained. Moreover, a black image (light transmittance: substantially 0%, applied voltage: substantially 0 V) shows no change during the application of voltage and the absence of applied voltage, and the display state is retained.
For the liquid crystal panel, a change in the light transmittance after removal of voltage is measured with time, and the measurement results are shown in FIG. 2. As shown in FIG. 2(a), a 5V, 100 μs pulse wave voltage is applied to the liquid crystal panel, and the light transmittance is measured with time. FIG. 2(b) shows the measured light transmittance by plotting the time (ms) on the abscissa and the light transmittance (arbitrary unit) on the ordinate. It can be understood that the light transmittance increases abruptly at the moment the voltage is applied and then attenuates gradually, but the attenuation is not seen 100 ms after the removal of voltage and the liquid crystal panel maintains a certain light transmittance.
It can be understood from the above description that the ferroelectric liquid crystal has the memory function, and even when the applied voltage is removed, the liquid crystal molecules maintain the previous state without moving from the stable position before the removal of the applied voltage to the other stable position. Thus, in a liquid crystal display device using a ferroelectric liquid crystal having such a memory function, when a voltage corresponding to the display information for one screen is applied once, a certain display corresponding to the applied voltage can be maintained without continuing the application of voltage, until a voltage corresponding to the display information for the next screen is applied. Consequently, it is possible to retain the display without applying the voltage, thereby enabling a reduction in power consumption.